The present invention relates to a scanning optical system for forming a beam spot scanning on a scan target surface such as a surface of a photoconductive drum.
As is well known, scanning optical systems are installed in a variety of printing devices such as laser beam printers, a photo plotter, fax machines and copy machines. The scanning optical system dynamically deflects a laser beam (which has been on-off modulated according to image information) by use of a deflecting system such as a rotating polygon mirror, and converges the dynamically deflected laser beam on the scan target surface by use of an imaging optical system, by which the scan target surface is scanned with a spot beam. With this structure, an electrostatic latent image composed of a plurality of dots is drawn on the scan target surface.
In the following description, a direction in which the beam spot moves (i.e., a direction in which a scan line extends) will be referred to as a main scanning direction. A direction in which the scan target surface moves with respect to the scan line, I.e., a direction of rotation of the photoconductive drum will be referred to as an auxiliary scanning direction. Further, shapes of optical elements, directions of powers of the optical elements and the like are described with reference to the main and auxiliary scanning directions on the surface to be scanned. That is, if an optical element is described to have a refractive power in the main scanning direction, the power affects the beam in the main scanning direction on the scan target surface regardless of the orientation of the element.
In order to reduce manufacturing cost of the scanning optical system, plastic lenses are frequently used in the imaging optical system of the scanning optical system. In general, such a plastic lens provided in the scanning optical system is not covered with an antireflective coating because cost for covering the plastic lens with the antireflective coating is typically higher than cost for covering a glass lens with the antireflective coating.
When the plastic lens without the antireflective coating is used in the scanning optical system, a problem that undesired reflections are caused and a portion of the undesired reflections-reaches the scan target surface as a ghost light beam may arise.
Each of Japanese Patent Provisional Publications No. HEI 05-346553 and No. HEI 07-287180 discloses a scanning optical system configured to prevent the ghost light beam from reaching the scan target surface. In the scanning optical system disclosed in the publications, an optical axis of at least one lens of an imaging optical system is tilted and/or shifted with respect to an optical axis of another lens of the imaging optical system.
To downsize the scanning optical system, it is required that space between lenses in the imaging optical system of the scanning optical system is narrow. In order to remove the ghost light beam when the space between the lenses in the optical system is narrow, the amount of tilting and the amount of shifting of the lens in the imaging optical system is required to be relatively large.
In general, such a lens of the imaging optical system is configured to have a frame (having, for example, a prismatic form or a cylindrical form) surrounding a fringe portion of the lens for mounting on a housing of the scanning optical system. When it is required that the lens surface is tilted or shifted with respect to the optical axis of another lens for elimination of the ghost light beam, the lens is required to have a configuration in which optical surface reference axes of both lens surfaces of the lens are tilted or shifted with respect to a center axis of the frame of the lens due to a requirement concerning assembly of the scanning optical system. The term “optical surface reference axis” means an axis passing through an origin point that is set when a shape of a lens surface is described in a mathematical expression. Therefore, when the lens surface is a rotationally symmetrical surface, the optical surface reference axis is equal to a rotational axis of the lens surface.
Meanwhile, in the development stage of a mold of a plastic lens, the following processes are required to mass-produce plastic lenses having high accuracy. Firstly, accuracy of a shape of a lens surface of the plastic lens molded by an initial mold is evaluated by comparing the shape of the lens surface with its theoretical shape. In this stage, it is required to find out an origin point (i.e., an intersection of the lens surface and its optical surface reference axis) of the lens surface molded by the initial mold.
Secondly, based on a result of the evaluation of accuracy of the shape of the lens surface, the initial mold is subjected to an additional cutting process to enhance accuracy thereof. By repeating the evaluation process and the additional cutting process, a mold for mass-production of the plastic lens having high accuracy can be achieved.
Although the above mentioned additional cutting process is necessary to enhance the accuracy of the mold, there is a case where the origin point of the lens surface shifts in the auxiliary scanning direction by the additional cutting process of the mold. For this reason, a considerably sophisticated technique is required to develop the mold of the plastic lens, which also increases manufacturing cost of the plastic lens.
In a case where the amounts of shifting of the optical surface reference axes of both lens surfaces of the plastic lens with respect to the center axis of the frame of the plastic lens are larger than half of a width of a mirror-finished surface area of the plastic lens in the auxiliary scanning direction, an additional problem that the origin points of the lens surfaces can not be found out occurs. When the origin point of the lens surface can not be found out, a special technique is required to evaluate the lens surfaces, which also increases manufacturing cost of the plastic lens.